The role of the local environment on the structural heterogeneity of carotenoid ß-ionone rings.

Our analysis of the X-ray crystal structure of canthaxanthin (CAN) showed that its ketolated ß-ionone rings can adopt two energetically equal, but structurally distinct puckers. Quantum chemistry calculations revealed that the potential energy surface of the ß-ionone ring rotation over the plane of the conjugated π-system in carotenoids depends on the pucker state of the ß-ring. Considering different pucker states and ß-ionone ring rotation, we found six separate local minima on the potential energy surface defining the geometry of the keto-ß-ionone ring-two cis and one trans orientation for each of two pucker states. We observed a small difference in energy and no difference in relative orientation for the cis-minima, but a pronounced difference for the position of trans-minimum in alternative pucker configurations. An energetic advantage of ß-ionone ring rotation from a specific pucker type can reach up to 8 kJ/mol ([Formula: see text]). In addition, we performed the simulation of linear absorption of CAN in hexane and in a unit cell of the CAN crystal. The electronic energies of [Formula: see text] transition were estimated both for the CAN monomer and in the CAN crystal. The difference between them reached [Formula: see text], which roughly corresponds to the energy gap between A and B pucker states predicted by theoretical estimations. Finally, we have discussed the importance of such effects for biological systems whose local environment determines conformational mobility, and optical/functional characteristics of carotenoid.

Excitation wavelength- and intensity-dependent stepwise two-photon-induced photochromic reaction.

The photochromic molecules showing wavelength-selective or light intensity-dependent photoresponse are receiving increased attention in recent years. Although a photoswitch with a single chromophore can control the ON and OFF states of a function, that consisting of multi-chromophores would be useful for the specific control in complex systems. Herein, we designed stepwise two-photon induced photochromic molecules (PABI-PIC and PABI-PIC2) consisting of two different photochromic units (PABI and PIC). One-photon absorption reaction in the UV light region of PABI-PIC generates the short-lived transient biradical (BR) that absorbs an additional photon in the visible and UV light region in a stepwise manner to produce the two-photon photochemical product, the quinoidal species (Quinoid). The photochromic properties of these transient species are completely different in color and fading speed. In addition, PABI-PIC also shows the excitation wavelength-dependent photochromism because the excited states of the PABI and PIC units are electronically orthogonal. Therefore, the stepwise photochromic properties of PABI-PIC are easily controlled depending on the excitation light intensity and wavelength. These molecular designs are important for the development of advanced photoresponsive materials.